Stator winding bar vibration
- If winding bars are loose , they will start vibrating due to the high forces (kilonewtons – tens of kilonewtons ) acting on them due to the magnetic field of the machine
- The forces are highest in slots with both bars in the same winding phase, where the force affects in slot direction towards the slot bottom
- The downward force is pulsating at 2 x power frequency (100 Hz or 120 Hz) and it is proportional to the square of the stator current
- Eventually a vibrating bar will almost certainly fail due to the punctured main insulation . The process may take years but not decades.
- Main insulation puncture is caused by 1) mechanical wear possibly combined with 2) strong classic PD caused by the damaged slot corona protection and/ or 3) vibration sparking erosion